Let them fight unreasonable CR melee brutes. It will still be a fair fight, and they will enjoy using their characters to their fullest extent.
If you do want to stretch them, add a 1st level kobold wizard to a weak encounter. Give the wizard spells like Grease or Daze. They'll notice that the wizard is punching above his weight.
Then make sure they get missile weapons in their loot (as said before). Put them up against a group of these wizards, with a chasm seperating them from the PCs (with a path round which would 'waste' 3 or so combat rounds). Give the wizards no real damage spells.
This is an encounter where using bows will finish it in a round or two. Trying to melee the kobolds will show them the flaws in their tactics (since the kobolds can grease the path!), but won't TPK them.
